For author affiliations and current author addresses, see end of text.
Acknowledgments: The authors thank Veronica Walton and Parvccn Vohra for assistance with the stable isotope analyses.
Grant Support: By grants from the U.S. Public Health Service; grants RO1-DK 49230 and RO1-DK 33649; GCRC grants MO1-RR00827, MO1-RR00125, and MO1-RR00043; and Warner Lambert Co. Dr. Shulman is an investigator of the Howard Hughes Medical Institute.
Requests for Reprints: David G. Maggs, MD, Yale University School of Medicine, Box 208020, New Haven, CT 06520-8020.
Current Author Addresses: Drs. Maggs, Cline, Inzucchi, Silver, and Shulman: Yale University School of Medicine, Box 208020, New Haven, CT 06520-8020.